78. In the “inside” region between the plates, the individual fields (given by Eq. 24.13) are in the same

direction (

−ˆi):

E

in

=

−

50

× 10

−9

2ε

0

+

25

× 10

−9

2ε

0



ˆi =

−4.2 × 10

3

ˆi

in SI units (N/C or V/m). And in the “outside” region where x > 0.5 m, the individual fields point in
opposite directions:

E

out

=

−

50

× 10

−9

2ε

0

ˆi+

25

× 10

−9

2ε

0

ˆi =

−1.4 × 10

3

ˆi .

Therefore, by Eq. 25-18, we have

∆V =

−



0.8

0

E

· d
s = −



0.5

0

|

E

|

in

dx

−



0.8

0.5

|

E

|

out

dx

=

−



4.2

× 10

3



(0.5)

−



1.4

× 10

3



(0.3)

=

2.5

× 10

3

V .